I'm pretty sure sanders is the most left of all the dems. The rest of them were more center-left. If he's center left, then which dem is the farthest left?
Left and right are relative, obviously. If we go according to the original definition of being opposed to the ancient regime in France then that would make essentially the entire United States decidedly left-wing. That isn't very useful.
You do have far-left organisations in the US, though. There's the CPUSA and the PSL, for example.
Just because electoral politics in a particular country lack one end of the spectrum doesn't mean the spectrum doesn't exist.